We have continued the description of the human UGT1 locus which has taken an unexpected turn as we established and used unique intronic probes and as we developed a universal UDP-glucuronosyltransferase (transferase) probe. The description of this locus has been critical in determining defects in patients with Crigler-Najjar(CN) diseases. The original UGT1A-UGT1F gene complex, recently renamed UGT1A1- UGT1A6, codes for bilirubin, bilirubin-like, and phenol transferases. In the 5' region of the locus, six different exons 1, each with an upstream promoter and each encoding the amino terminus of an isoform, are arrayed in series with 4 common exons encoding six identical carboxyl termini in the 3' region. We have extended the locus by identifying, sequencing, and mapping UGT1A7-UGT1A12P genes. Out of the first 12 genes, three are pseudo. An additional 11-13 exons 1 have been detected by using the universal exon probe; this finding is predicted to generate a total of 23-25 exons 1 at this locus. Exons 1A7-1A12P are 86- 91% identical to each other and distantly related, 50-53 % identical, to the first six exons 1. The 12 different TATA box elements upstream of exons 1A1-1A12P already described is either TAATAA, AA(TA)7A, AAT9AAAT, or AT14AT. Promoter mutations have been uncovered in the genome of CN-I and CN-II individuals; sequence data showed a TA insertion at the AA(TA)7A box upstream of the UGT1A1 bilirubin transferase gene. Transcriptional activity of a reporter gene fused to the 1A1 upstream region containing either a TA insertion or deletion was inhibited 80% and 35%, respectively, using HepG2 cells. Mobility shift assays with either radiolabeled AA(TA)6A, AA(TA)7A or AA(TA)8A showed that a hepatocyte-specific nuclear protein(s), at low (< 0.5 microgram) concentrations, binds to the AA(TA)6A and AA(TA)8A mutant boxes but not to the wild type one. Additionally, the missense mutation in the CN-1 (R336W) or CN-II (I294T) patient was completely or partially inactivating, respectively. Messenger RNAs coding for the UGT1A7-UGT1A9 proteins are expressed mainly in colon, small intestine, bladder, skeletal and heart muscle; UGT1A10 is only in heart muscle. The UGT1A8 isoform glucuronidates primarily complex 4-hydroxy substituted flavones and anthroquinones and certain types of 2, 4 substituted phenols. Polymorphisms in the population have been uncovered in the UGT1A6 gene. Some 33/100 individuals randomly selected are heterozygous for both a T181A and R184S mutation in one allele, UGT1A6*2; we detected no homozygotes. UGT1A6*2-encoded transferase exhibited 28% to 69 % normal activity for experimental and therapeutic drugs. Hence, the expansive human UGT1 locus encodes some 23-24 genes with independent regulation and tissue distribution to allow for a wide range in protection against chemicals we are exposed to through our environment and diet.